Don't assume /bin/bash is available on every system. (#1223)

Co-authored-by: MikaelUrankar <mikael.urankar@gmail.com>

scripts/filter_profile.rs

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 #![forbid(unsafe_code)]/* This line is ignored by bash
 # This block is ignored by rustc
 pushd $(dirname "$0")/../

scripts/setup_rust_fork.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 set -e
 
 ./y.rs build --no-unstable-features

scripts/test_rustc_tests.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 set -e
 
 cd $(dirname "$0")/../